In a refracting telescope, the lens is what bends the light and makes it possible to view faraway objects as if they are close up. One of the lenses in this type of telescope is usually larger than the other and they work together to magnify the object you are looking at bringing it into closer view.
Some people consider the telescope lens the eyepiece that you look through and it is a type of lens as well. When you are talking about the eyepiece, it is probably the most important part of the telescope aside from the device itself.
Most telescopes come with one eyepiece (low power), while some telescopes come with none. Therefore, you may have to purchase eyepieces so that you can vary the magnification of your telescope. Eyepieces come in many designs and different magnifications. Which one you choose is a matter of personal preference.
The designs vary in terms of the number and types of lenses, or elements, they use. Eyepieces should be evaluated for the following:
* optical quality
* field size
* lack of aberrations (chromatic aberrations, ghost images)
* eye relief (distance from the focal point, your eye, to the lens — especially important for eyeglass wearers)
* barrel size – 0.965 inches, 1.25 inches, 2 inches
The oldest telescope lens designs are Huygens and Ramsden. They are often included with cheaper, department store type telescopes and aren’t very good lenses to use if you are an experienced astronomer. They often have many chromatic aberrations or circles of light around bright objects which can be distracting for some people.
A great telescope lens for both experienced as well as amateur sky gazers would be an orthoscopic design. They have four lenses in the eyepiece and a 45-degree apparent field of view which is somewhat narrow. This optical design, however, gives a crisp view, has good eye relief, and is considered excellent for planetary viewing.
An economical choice for a telescope eyepiece is the Barlow lens. They can increase magnification and provide better eye relief with an existing eyepiece. The eyepiece fits into the lens which then fits into the eyepiece holder. The Barlow lens ranges in price from $30 to $70.
When you are looking at telescopes, look for a lens that is adjustable or which can be adapted for either star gazing or planet viewing, and then enjoy the view!